Plus-fraction corrections for heavy hydrocarbon liquids

ABSTRACT

A method of determining the plus-fraction correction for a heavy oil sample including analyzing the heavy oil sample using gas chromatography, selecting an elution ratio based on the percentage of plus-fraction elution, applying the elution ratio to provide a maximum carbon number and a plus-fraction correction to compensate for the non-elution of the plus-fraction. An elution ratio as a function of the percentage of C 60+  elution is provided. A corrected molecular weight for the C 60+  fraction (MW C60+ ) and the C 90+  fraction (MW C90+ ) as well as the total oil (MW) is provided.

BACKGROUND

Field

The present disclosure relates generally to analysis of heavy oil. More particularly, the present disclosure relates to chromatography of heavy oil.

Description of the Related Art

Chromatography includes the analysis of a compound to determine the identification and relative amount of the components. A sample of the compound is heated in a chromatographic column and the components detected at a detector.

Standards exist for the analysis of petroleum fractions including, for example, ASTM International standards relating to test methods for boiling range distribution of petroleum fractions by gas chromatography.

The quantification of heavy oil compositions is challenging for several reasons, which may include oxidation, emulsions, or impurities such as basic sediment and water (BS&W), or contamination.

However, one of the more difficult problems to resolve is the degree of non-elution, meaning the portion of the heavy oil that is not eluted, and thus does not arrive at the detector but instead remains behind in the chromatographic column.

It is, therefore, desirable to provide a method for chromatography of heavy oils.

SUMMARY

The technique used for heavy oil characterization is based upon treatment of the heavy oil as two portions. A first portion includes components having a carbon number up to and including a full elution carbon number, and a second portion includes components having a carbon number greater than the full elution carbon number. In an embodiment disclosed the full elution carbon number is 60, and the first portion includes components having a carbon number up to and including C₆₀, and the second portion includes components having a carbon number C₆₀₊. The first portion of the heavy oil may be characterized by typical means, for example gas chromatography. The second portion of the heavy oil may be characterized by the methods described herein. The characterization of the first portion and the second portion may be combined to provide a characterization for the total oil.

In an embodiment disclosed, a method for determining the area fraction of the heavy oil that is not eluted and, since area is proportional to mass in a flame ionization detector (FID), this technique provides the mass fraction non-eluted.

In a first aspect, the present disclosure provides a method of determining a plus-fraction correction for a heavy oil including obtaining an analysis of the heavy oil using gas chromatography to provide a highest detected carbon number (C_(EMAX)), a total eluted area (A_(Etotal)), and a C₆₀₊ eluted area (A_(EC60+)), selecting an elution ratio (R) to compensate for the non-elution of a plus-fraction of the heavy oil, determining a corrected plus-fraction area (A_(C60+)) from the elution ratio (R) and the C₆₀₊ eluted area (A_(EC60+)) according to the formula A_(C60+)=A_(EC60+)×R, and determining a C₆₀ area (A_(C60)) and maximum carbon number (C_(MAX)) for the heavy oil according to the formulas A_(EC60+)=0.50×(C_(EMAX)−60)×A_(C60) and A_(C60+)=0.50×(C_(MAX)−60)×A_(C60). The analysis of the heavy oil using gas chromatography may, for example, include running a sample of the heavy oil thorough a gas chromatograph or using results from a previous gas chromatograph run for the heavy oil.

In an embodiment disclosed, the elution ratio (R), is determined according to the formula R=0.0493 In (A_(C60+)/A_(Etotal)×100)+1.1746.

In an embodiment disclosed, the method further includes determining a slope of a non-elution line according to the formula slope=(0−A_(C60))/(C_(MAX)−60), and determining at least one area fraction for a carbon number (C_(Ni)) according to the formula A_(CNi)=A_(C60)+slope×(C_(Ni)−60).

In an embodiment disclosed, the method further includes determining an area fraction A_(CNi), for each carbon number (C_(Ni)) between C₆₀ and C_(MAX).

In an embodiment disclosed, the method further includes determining a mole fraction for each carbon number (C_(Ni)) between C₆₀ and C_(MAX).

In an embodiment disclosed, the method further includes summing the molecular weight fraction for each carbon number (C_(Ni)) between C₆₀ and C_(MAX) to determine a C₆₀₊ molecular weight (MW_(C60+)) of the heavy oil.

In an embodiment disclosed, the method further includes summing the molecular weight fraction for each carbon number (C_(Ni)) between C₉₀ and C_(MAX) to determine a C₉₀₊ molecular weight (MW_(C90+)) for the heavy oil.

In an embodiment disclosed, the method further includes summing the molecular weight fraction for each carbon number (C_(Ni)) between C_(MIN) and C_(MAX) to determine a molecular weight (MW) of the heavy oil.

In an embodiment disclosed, C_(MIN) equals C₃.

In an embodiment disclosed, the method further includes determining a plus-fraction area, between C₆₀ and C_(MAX), according to the formula A_(C60+)=∫A_(CNi)*dC_(N).

In an embodiment disclosed, the method further includes determining a plus-fraction area, between C₆₀ and C_(MAX), according to the formula A_(C60+)=A_(C60)×C_(Ni)+slope (C_(Ni) ²/2−60×C_(Ni)).

In an embodiment disclosed, A_(C60+)/A_(Etotal)×100 is between 1 percent and 100 percent. In an embodiment disclosed, A_(C60+)/A_(Etotal)×100 is between 2 percent and 40 percent. In an embodiment disclosed, A_(C60+)/A_(Etotal)×100 is between 2 percent and 30 percent.

A computer or system having one or more processors may be utilized to make the calculations of the present disclosure. In an embodiment, a non-transitory computer-readable medium has computer-readable code embodied therein. The computer-readable code is executable by a processor to use an analysis of the heavy oil using gas chromatography to provide a highest detected carbon number (C_(EMAX)), a total eluted area (A_(Etotal)), and a C₆₀₊ eluted area (A_(EC60+)), and selecting an elution ratio (R) to compensate for the non-elution of a plus-fraction of the heavy oil, automatically determining a corrected plus-fraction area (A_(C60+)) from the elution ratio (R) and the C₆₀₊ eluted area (A_(EC60+)) according to the formula A_(C60+)=A_(EC60+)×R, and determining a C₆₀ area (A_(C60)) and maximum carbon number (C_(MAX)) for the heavy oil according to the formulas A_(EC60+)=0.50×(C_(EMAX)−60)×A_(C60) and A_(C60+)=0.50×(C_(MAX)−60)×A_(C60).

Other aspects and features of the present disclosure will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present disclosure will now be described, by way of example only, with reference to the attached Figures.

FIG. 1 is a sample chromatogram for a heavy oil sample;

FIG. 2 is a correlation of an elution ratio R versus percent C₆₀₊ eluted;

FIG. 3 is a detail of the C₆₀₊ portion of FIG. 1, illustrating the relationship between an area eluted for C₆₀₊ (A_(EC60+)) and an area non-eluted for C₆₀₊ (A_(NEC60+)), and

FIGS. 4A-4C is a table for an example of the present disclosure.

DETAILED DESCRIPTION

Generally, the present disclosure provides a method for quantification of heavy oil which compensates for the non-elution of a portion of the heavy oil being tested.

Chromatography

Referring to FIG. 1, chromatography provides identification and quantification of the components of a compound, for example carbon atoms in heavy oil. The relative amount of each component, for example C_(MIN) through C_(EMAX) is indicated by the area of each component. The C_(MIN) compound being the compound with the lowest carbon number eluted that registers a non-zero amount on the chromatograph. C_(MIN) may be typically about C₅, as shown in FIG. 1 for a heavy oil at atmospheric conditions. The compound C_(EMAX) being the compound with the highest carbon number eluted that registers a non-zero amount on the chromatograph. Components undoubtedly exist beyond C_(EMAX), for example up to and including C_(MAX), but are not eluted and therefore are not revealed by the chromatograph.

On the basis of poly-wax quantitative sample containing 16 components up to C₆₀, it is understood that using a high temperature gas chromatograph which operates up to 400° C. and uses helium as a carrier gas, that 100 percent elution is achieved at least up to and including C₆₀.

However, non-elution may start at C₆₁ or beyond but no quantitative sample exists where the degree of elution can be quantified. Therefore, in an embodiment disclosed, the full elution carbon number, being the highest carbon number where substantially one-hundred (100) percent elution is known to be achieved, is selected as C₆₀.

Elution Ratio R

In an embodiment disclosed, a method for determining the area fraction of the heavy oil non-eluted is provided, and since area is proportional to mass in a flame ionization detector (FID), this technique provides the mass fraction non-eluted.

In an embodiment disclosed, a non-eluted correction may be expressed as: A _(NEC60+) =A _(EC60+)(R−1)  Eq. (1)

In Eq. (1), A_(NEC60+) is the area of the C₆₀₊ not eluted and A_(EC60+) is the area of the eluted C₆₀₊. The elution ratio, R is equal to the total amount of hydrocarbons (A_(EC60+) plus A_(NEC60+)) over the amount of hydrocarbons eluted (A_(EC60+)): R=(A _(EC60+) +A _(NEC60+))/A _(EC60+)  Eq. (2)

Eq. (2) may be rewritten by substituting the total area A_(C60+) for the area eluted A_(EC60+) plus the area non-eluted A_(NEC60+): R=A _(C60+) /A _(EC60+)  Eq. (3)

The elution ratio, R, may be represented as: R=C ₆₀₊Total/C ₆₀₊Eluted  Eq. (4)

Determination of Elution Ratio, R

The C₆₀₊ total area was derived from multiple blank runs performed after a sample of heavy oil was provided into the gas chromatograph column. The total area of the blanks cumulated and added to the eluted C₆₀₊ area divided by the eluted C₆₀₊ provides an elution ratio, R.

As an example, a heavy oil of known composition, labeled C-E006 was sampled in the gas chromatograph. This sample indicated an eluted C₆₀₊ of 3.69569, but was known to have a total C₆₀₊ of 5.3272, so a non-eluted C₆₀₊ of 1.6315 may be determined. Applying Eq. (4) provides R=1.441.

In an embodiment disclosed, a correlation is provided to determine and use the elution ratio, R for a sample heavy oil of unknown composition.

Referring to FIG. 2, the elution ratio, R is shown in correlation with percent C₆₀₊ eluted for wide variety of heavy oils, that is percent C₆₀₊ eluted=A_(EC60+)/A_(Etotal)×100. In an embodiment disclosed, the logarithmic curve fit correlation is represented as: R=0.0493 ln(percent C ₆₀₊ Eluted)+1.1746  Eq. (5)

A lower limit of 0.03 percent C₆₀₊ may be used, in order to limit the R to 1.00, i.e., using Eq. (5) 0.0493 In (0.03)+1.1746 equals substantially 1.00.

With a sample of heavy oil of unknown composition, one can conduct a gas chromatograph run, and using the percent C₆₀₊ eluted (A_(EC60)/A_(Etotal)*100), determine the elution ratio, R, for the sample. The elution ratio, R determined in this manner may be used to compensate for the non-elution.

Referring to FIG. 3, a detail of the chromatograph of FIG. 1, from C₆₀ and higher is shown.

With the elution ratio, R and utilizing a linear elution phenomenon in the range of the components in question, C₆₀ through C_(EMAX) and C₆₀ through and C_(MAX), two linear equations result, where the area of the C₆₀ component is determined and subsequently the maximum carbon number (CN) of the non-eluted fraction is determined.

A_(C60)

Using the A_(EC60+) value and C_(EMAX) value measured from the heavy oil sample using the gas chromatograph, the area for the C₆₀ component, being A_(C60), may be determined from: A _(EC60+)=0.50×(C _(EMAX)−60)×A _(C60)  Eq. (6)

C_(MAX)

Then A_(C60+) may be determined using R and A_(EC60+) using Eq. 3, from above. Finally, C_(MAX) may be calculated using the A_(C60+) and A_(C60) values from above, and equation (7) the maximum carbon number (CN) C_(MAX) of the non-eluted fraction: A _(C60+)=0.50×(C _(MAX)−60)×A _(C60)  Eq. (7)

Components

The equation of the gas chromatograph area for each component, including eluted and non-eluted contributions, may be derived, and from that the total area of the C₆₀₊ may be determined.

Knowing the mass fraction for every component in the C₆₀₊ range, and assuming a molecular weight (MW) increment of 14.0267 per carbon number (C—H₂), as in an alkane C_(n)H_(2n+2), the renormalized C₆₀₊ can be used to compute the C₆₀₊ molecular weight.

The total A_(C60+) is determined by integrating the area over the range of carbon numbers in the range.

Example 1: Heavy Oil Sample WL1

Referring to FIGS. 1, and 3, in an example, a heavy oil sample WL1 is analyzed using a high temperature gas chromatograph column, which operates at up to 400° C. and uses helium as a carrier gas. The full elution carbon number is selected as 60, that is, C₆₀.

The gas chromatograph indicates the presence of hydrocarbons up to C₁₀₈. That is, the highest degree carbon component showing any significant degree of detected elution is C₁₀₈. Therefore C_(EMAX)=108. The chromatograph further indicates that the A_(EC60+)=1,908,000 (area for eluted C₆₀₊) and that A_(Etotal)=14,290,000 (total area).

Elution Ratio, R

In an embodiment, disclosed is a correlation between the elution ratio R, and the eluted C₆₀₊ area A_(EC60+) relative to the eluted total area A_(Etotal). In this example, A_(EC60+)/A_(Etotal)=1,908,000/14,290,000=0.1335=13.35 percent.

Referring to FIG. 2, or Eq. (5), 13.35 percent provides an elution ratio, R of =0.0493 In (13.35)+1.1746=1.3024.

Using the A_(EC60+) of 1,908,000 measured from the gas chromatograph and the elution ratio R=1.3024, A_(C60+) is determined according to Eq. (3), which may be rearranged: A _(C60+) =A _(EC60+) ×R  Eq. (8)

Solving this equation provides A_(C60+)=1,908,000×1.3024=2,484,979, which as above, is the total area of eluted and non-eluted for C₆₀₊.

Quantification

Using Eq. (6) above with the A_(EC60+) and C_(EMAX) data from the chromatograph provides the area of the C₆₀ component: 1,908,000=0.50×(108−60)×A _(C60)  from Eq. (6)

In this example, with A_(EC60+)=1,908,000 and C_(EMAX)=108 from the chromatograph, Eq. (6) provides A_(C60)=79,500.0.

C_(MAX) is determined according to Eq. (7) above using A_(C60+) from Eq. (8): 2,484,979=0.50×(C _(MAX)−60)×79,500  from Eq. (7)

Solving Eq. (7) with A_(C60)=2,484,979 (this is the A_(C60+) based on the elution ratio, R) and A_(C60)=79,500.0, provides C_(MAX)=122.52. As the carbon number (CN) must be an integer, C_(MAX) is rounded up to the next higher integer. The calculated C_(MAX)=122.52 is thus rounded up to C_(MAX)=123.

Corrected Profile

Referring to FIG. 3, the slope of the non-eluted line is: Slope=(0−A _(C60))/(C _(MAX)−60)  Eq. (9)

As the values for A_(C60), C_(MAX), and C_(EMAX) are known, Eq. (9) can be solved: Slope=(0−79500.0)/(123−60)=−1261.9048  from Eq. (9)

The slope may then be used to determine the actual area for any component from C₆₀ through C_(MAX), which takes into account the elution ratio, R, and thus compensates for the non-elution of the heavy oil: A _(CNi) =A _(C60)+Slope×(C _(Ni)−60)  Eq. (10) A _(CNi)=79,500−1,261.9048×(C _(Ni)−60)  from Eq. (10)

For example, the area A_(C100) for C₁₀₀ equals, according to Eq. (7): 79,500−1,261.9048×(100−60)=29,023.8  from Eq. (10)

This may be calculated between 0 to C_(MAX)). The equation of the gas chromatograph area for each component, including eluted and non-eluted contributions, is derived from which the total area of the C₆₀₊ is readily available. This calculation can be performed for any of CNi, where in this case i is between 60 and C_(MAX) (in this example C_(MAX)=123) to provide the area, and thus mass fraction, or mole fraction, or both for any component.

Mass Fraction (MF)

One can also sum, totalize, or integrate the area A_(CNi) from C₆₀ to C_(MAX), to determine the total area: A _(C60+) =∫A _(CNi) *dC _(N), between C ₆₀ and C _(MAX).  Eq. (11)

Using Eq. (10) above and performing the integration, Eq. (11), provides the formula: A _(C60+) =A _(C60) ×C _(Ni)−1261.9048(C _(Ni) ²/2−60C _(Ni))  Eq. (12)

Calculating A_(C60+) with C_(Ni) between 60 and C_(MAX), provides A_(C60)=2,504,249.92.

There is a small difference between the A_(C60+) of 2,504,249.92 calculated here and the A_(C60+) of 2,484,979 calculated at Eq. (8) due to the rounding up of the C_(MAX) to an integer, in this example from 122.52 to 123.

Using the A_(Etotal) above from the gas chromatograph, the mass fraction of C₆₀₊ (MF_(C60+)) may be determined: MF _(C60+) =A _(C60+)/(A _(Etotal) +A _(NEC60+))  Eq. (13)

Using the A_(C60+) calculated above, the A_(Etotal) from the gas chromatograph, and using the A_(NEC60+)=A_(EC60+)(R−1) from Eq. (1), the mass fraction of C₆₀₊ is: MF _(C60+) =A _(C60+)/(A _(Etotal) +A _(EC60+)×(R−1))  from Eq. (13)

Using A_(NEC60+)=A_(EC60+)(R−1) from equation (1), and the previously determined A_(EC60+) and R, solving Eq. (13) provides: MF _(C60+)=2,504,249.92/(14,290,000+1,908,000×(1.3024−1)) MF _(C60+)=0.1684  from Eq. (13)

This indicates that, in this example, the mass fraction for C₆₀₊ is 0.168.

Molecular Weight

Referring to FIGS. 4A-4C, a table provides an example determination of the molecular weight C₆₀₊(MW_(C60+)) and molecular weight C₉₀₊(MW_(C90+)) using the methods described herein. Such quantifications are commonly used for calculations or analysis, for example computer simulations of heavy oil reservoirs.

For each carbon number, C_(Ni), the molecular weight, area, mass fraction, moles, C₆₀₊ mole fraction, and C₉₀₊ mole fraction may be determined.

It is assumed that the hydrocarbon is an alkane, i.e. C_(n)H_(2n+2). Using a molecular weight of C of 12.01078 g/mol and a molecular weight of H of 1.007947 g/mol, for example, C₁₀₀, as C₁₀₀H₂₀₂, would have a molecular weight of 100×12.01078+202*1.007944=1,404.68 g/mol.

The area for C₁₀₀, as A_(C100) may also be calculated, as above, and as shown in FIGS. 4A-4C, A_(C100) is 29,023.8.

The areas for each of C_(m) may be totaled. In FIGS. 4A-4C, the total area for C₆₀ through C₁₂₃ is 2,504,250. Again, there is a small difference between the A_(C60+) of 2,504,250 calculated here and the A_(C60+) of 2,504,250 and the A_(C60+) of 2,484,979 calculated earlier.

For each C_(Ni), the mass fraction may be determined from the area divided by the total area. In the example, C₁₀₀, having an area A_(C100) of 29,023.8 divided by the total area of 2,544,000 provides the mass fraction of C₁₀₀. As shown the mass fraction of C₁₀₀ is 29,023.8/2,544,000=0.0114. The number of moles of C₁₀₀ is the mass fraction divided by the molecular weight, that is 0.0114/1,404.68.45=8.1219×10⁻⁶. This is repeated for each C_(Ni) between C₆₀ through C_(MAX).

The number of moles for C₆₀₊ and C₉₀₊ may be summed. As shown in FIGS. 4A-4C, the total C₆₀₊ is 9.114×10⁻⁴ and the total for C₉₀₊ is 1.9796×10⁻⁴.

The mole fraction may be calculated for each C_(Ni), for each of C₆₀₊ and C₉₀₊. As shown in the example in FIGS. 4A-4C, the mole fraction for C₁₀₀ is 0.0089 for C₆₀₊ and 0.0410 for C₉₀₊.

The mole fraction for each C_(Ni) may be multiplied by the molecular weight, and the product sum provides the molecular weight. In the example shown in FIGS. 4A-4C, the molecular weight of the C₆₀₊ components is 1,097.20 g/mol, and the molecular weight of the C₉₀₊ components is 1,405.69 g/mol.

Total Oil

The composition of the whole oil may be provided using the complete elution of the C₅ to C₅₉ components from the gas chromatograph combined with the C₆₀₊ as determined in accordance with the present disclosure.

General

The technique can easily be generalized to provide the molecular weight and mass fraction for a selected plus fraction.

As used herein, heavy oil is defined as petroleum liquids having an API gravity of less than about 12° API (i.e. a density of greater than 986 kg/m³).

In an alternative embodiment, the quantification may be made using another selected plus-fraction, for example C₉₀₊ or another selected plus-fraction. As an example, another selected plus-fraction may be C₅₀₊, C₁₂₀₊ or C₁₀₀₊ or some other selected plus-fraction, in which case the selected plus-fraction would be substituted in the above description for C₆₀₊. That is, for example, the selected cutoff could be at other than C₆₀, with corresponding adjustments in the above method.

In the preceding description, for purposes of explanation, numerous details are set forth in order to provide a thorough understanding of the embodiments. However, it will be apparent to one skilled in the art that these specific details are not required.

The above-described embodiments are intended to be examples only. Alterations, modifications and variations can be effected to the particular embodiments by those of skill in the art without departing from the scope, which is defined solely by the claims appended hereto. 

What is claimed is:
 1. A method of determining composition for a heavy oil, the method comprising: analyzing a sample of a heavy oil with a gas chromatograph to obtain analysis results for the heavy oil, comprising: running the sample of the heavy oil through a gas chromatograph column, the sample including an eluted portion and a plus fraction portion; and eluting the eluted portion from the gas chromatograph column; determining a highest detected carbon number (C_(EMAX)), a total eluted area (A_(Etotal)), and a C₆₀₊ eluted area (A_(EC60+)) in the analysis results; setting an elution ratio (R) to compensate for non-elution of a plus-fraction of the heavy oil from the gas chromatograph according to a pre-established correlation between a ratio of the C₆₀₊ eluted area (A_(EC60+)) to the total eluted area (A_(Etotal)) for heavy oil samples and the elution ratio (R); determining a corrected plus-fraction area (A_(C60+)) from the elution ratio (R) and the C₆₀₊ eluted area (A_(EC60+)) according to the formula: A _(C60+) =A _(EC60+) ×R; and determining a C₆₀ area (A_(C60)) and maximum carbon number (C_(MAX)) for the heavy oil according to the formulas: A _(EC60+)=0.50×(C _(EMAX)−60)×A _(C60) and A _(C60+)=0.50×(C _(MAX)−60)×A _(C60).
 2. The method of claim 1, wherein the pre-established correlation is the formula: R=0.0493 ln(A _(C60+) /A _(Etotal)×100)+1.1746.
 3. The method of claim 2, further comprising: determining a slope of a non-elution line according to the formula: slope=(0−A _(C60))/(C _(MAX)−60); and determining at least one area fraction for a carbon number (C_(Ni)) according to the formula: A _(CNi) =A _(C60)+slope×(C _(Ni)−60).
 4. The method of claim 3, further comprising determining an area fraction A_(CNi) for each carbon number (C_(Ni)) between C₆₀ and C_(MAX).
 5. The method of claim 4, further comprising determining a mole fraction for each carbon number (C_(Ni)) between C₆₀ and C_(MAX).
 6. The method of claim 5, further comprising summing the molecular weight fraction for each carbon number (C_(Ni)) between C₆₀ and C_(MAX) to determine a C₆₀₊ molecular weight (MW_(C60+)) for the heavy oil.
 7. The method of claim 5, further comprising summing the molecular weight fraction for each carbon number (C_(Ni)) between C₉₀ and C_(MAX) to determine a C₉₀₊ molecular weight (MW_(C90+)) for the heavy oil.
 8. The method of claim 5, further comprising summing the molecular weight fraction for each carbon number (C_(Ni)) between C_(MIN) and C_(MAX) to determine a molecular weight (MW) for the heavy oil.
 9. The method of claim 8, wherein C_(MIN) equals C₃.
 10. The method of claim 2, further comprising determining a plus-fraction area, between C₆₀ and C_(MAX), according to the formula: A _(C60+) =∫A _(CNi) *dC _(N).
 11. The method of claim 2, further comprising determining a plus-fraction area, between C₆₀ and C_(MAX), according to the formula: A _(C60+) =A _(C60) ×C _(Ni)+slope(C _(Ni) ²/2−60×C _(Ni).
 12. The method of claim 2, wherein A_(C60+)/A_(Etotal)×100 is between 1 percent and 100 percent.
 13. The method of claim 12, wherein A_(C60+)/A_(Etotal)×100 is between 2 percent and 40 percent.
 14. The method of claim 13, wherein A_(C60+)/A_(Etotal)×100 is between 2 percent and 30 percent.
 15. A computer implemented method for determining composition for a heavy oil, the method comprising: obtaining gas chromatographic analysis results for a sample of a heavy oil, comprising: running the sample of the heavy oil through a gas chromatograph column, the sample including an eluted portion and a plus fraction portion; and eluting the eluted portion from the gas chromatograph column; determining a highest detected carbon number (C_(EMAX)), a total eluted area (A_(Etotal)), and a C₆₀₊ eluted area (A_(EC60+)) in the gas chromatographic analysis results for the heavy oil; setting an elution ratio (R) to compensate for non-elution of a plus-fraction of the heavy oil from a gas chromatograph used in obtaining the gas chromatographic analysis results according to a pre-established correlation between a ratio of the C₆₀₊ eluted area (A_(EC60+)) to the total eluted area (A_(Etotal)) for heavy oil samples and the elution ratio; determining a corrected plus-fraction area (A_(C60+)) from the elution ratio (R) and the C₆₀₊ eluted area (A_(EC60+)) according to the formula: A _(C60+) =A _(EC60+) ×R; and determining a C₆₀ area (A_(C60)) and maximum carbon number (C_(MAX)) for the heavy oil according to the formulas: A _(EC60+)=0.50×(C _(EMAX)−60)×A _(C60) and A _(C60+)=0.50×(C _(MAX)−60)×A _(C60).
 16. The method of claim 15, wherein the pre-established correlation is the formula: R=0.0493 ln(A _(C60+) /A _(Etotal)×100)+1.1746.
 17. A non-transitory computer-readable medium having computer-readable code embodied therein, the computer-readable code executable by a processor of a computing device to implement a method for determining composition for a heavy oil, the implemented method comprising: obtaining gas chromatographic analysis results for a sample of a heavy oil, comprising: running the sample of the heavy oil through a gas chromatograph column, the sample including an eluted portion and a plus fraction portion; and eluting the eluted portion from the gas chromatograph column; determining a highest detected carbon number (C_(EMAX)), a total eluted area (A_(Etotal)), and a C₆₀₊ eluted area (A_(EC60+)) in the gas chromatographic analysis results for the heavy oil; setting an elution ratio (R) to compensate for non-elution of a plus-fraction of the heavy oil from a gas chromatograph used in obtaining the gas chromatographic analysis results according to a pre-established correlation between a ratio of the C₆₀₊ eluted area (A_(EC60+)) to the total eluted area (A_(Etotal)) for heavy oil samples and the elution ratio (R); determining a corrected plus-fraction area (A_(C60+)) from the elution ratio (R) and the C₆₀₊ eluted area (A_(EC60+)) according to the formula: A _(C60+) =A _(EC60+) ×R; and determining a C₆₀ area (A_(C60)) and maximum carbon number (C_(MAX)) for the heavy oil according to the formulas: A _(EC60+)=0.50×(C _(EMAX)−60)×A _(C60) and A _(C60+)=0.50×(C _(MAX)−60)×A _(C60).
 18. The non-transitory computer-readable medium of claim 17, wherein the pre-established correlation is the formula: R=0.0493 ln(A _(C60+) /A _(Etotal)×100)+1.1746. 